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Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 II. Two-dimensional magnetic Doppler imaging in all four Stokes parameters
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, A123Article in journal (Refereed) Published
Abstract [en]

Aims. We present a magnetic Doppler imaging study from all Stokes parameters of the cool, chemically peculiar star HD24712. This is the very first such analysis performed at a resolving power exceeding 105. Methods. The analysis is performed on the basis of phase-resolved observations of line profiles in all four Stokes parameters obtained with the HARPSpol instrument attached at the 3.6 m ESO telescope. We used the magnetic Doppler imaging code INVERS 10, which allowed us to derive the magnetic field geometry and surface chemical abundance distributions simultaneously. Results. We report magnetic maps of HD24712 recovered from a selection of Fe I, Fe II, Nd III, and Na i lines with strong polarization signals in all Stokes parameters. Our magnetic maps successfully reproduce most of the details available from our observation data. We used these magnetic field maps to produce abundance distribution map of Ca. This new analysis shows that the surface magnetic field of HD24712 has a dominant dipolar component with a weak contribution from higher-order harmonics. The surface abundance distributions of Fe and Ca show enhancements near the magnetic equator with an underabundant patch at the visible (positive) magnetic pole; Nd is highly abundant around the positive magnetic pole. The Na abundance map shows a high overabundance around the negative magnetic pole. Conclusions. Based on our investigation and similar recent magnetic mapping studies that used four Stokes parameters, we present tentative evidence for the hypothesis that Ap stars with dipole-like fields are older than stars with magnetic fields that have more small-scale structures. We find that our abundance maps are inconsistent with recent theoretical calculations of atomic diffusion in presence of magnetic fields.

Place, publisher, year, edition, pages
2015. Vol. 573, A123
Keyword [en]
stars: chemically peculiar, stars: atmospheres, stars: abundances, stars: individual: HD24712, stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-251539DOI: 10.1051/0004-6361/201424559ISI: 000350806000023OAI: oai:DiVA.org:uu-251539DiVA: diva2:806524
Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2017-09-07Bibliographically approved
In thesis
1. Magnetic fields and chemical maps of Ap stars from four Stokes parameter observations
Open this publication in new window or tab >>Magnetic fields and chemical maps of Ap stars from four Stokes parameter observations
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Our knowledge of stellar magnetic fields relies almost entirely on circular polarization observations, which has historically limited our understanding of the stellar magnetic field topologies. Recently, it has become possible to obtain phase-resolved high-resolution spectropolarimetric observations in all four Stokes parameters for early-type magnetic stars. Interpretation of such observations with the Magnetic Doppler imaging technique has uncovered a new, previously unknown, level of complexity of surface stellar magnetic fields. This new insight is critical for understanding the origin, evolution and structure of magnetic fields in early-type stars.

In this study we observed the magnetic, chemically peculiar Ap stars HD 24712 (DO Eri, HR 1217) and HD 125248 (CS Vir, HR 5355) in all four Stokes parameters with the HARPSpol spectropolarimeter at the ESO 3.6-m telescope. The resulting spectra have high signal-to-noise ratio and superb resolving power, by far surpassing the quality of any existing stellar Stokes parameter observations.

We studied variation of the spectrum and magnetic observables of HD 24712 as a function of rotational phase (paper I). In the subsequent magnetic Doppler imaging investigation of this star, we interpreted the phase-resolved Stokes line profile observations (paper II). This analysis showed that HD 24712, unlike more massive Ap stars studied in all four Stokes parameters, has a dominant dipolar field component with a negligible contribution of small-scale magnetic structures. Simultaneously with magnetic mapping we derived surface abundance distributions of Fe, Nd, Na, and Ca.

Building upon the technique of Magnetic Doppler imaging, we developed the first three-dimensional abundance inversion code and applied it to reconstruct the abundance distributions of Fe and Ca in three dimensions in the atmosphere of HD 24712 (paper III).

We also performed Magnetic Doppler imaging analysis of the spectropolarimetric observations of HD 125248 (paper IV). The reconstructed detailed maps of the surface abundance distribution and magnetic field topology of HD 125248 revealed a magnetic field with significant deviations from the canonical dipolar field geometry, and strong surface abundance inhomogeneities for Cr and several rare earth elements.

We assessed our inversion results in the context of magnetic Doppler imaging studies of other magnetic, chemically peculiar Ap stars and latest theoretical research on the evolution and stability of magnetic fields in radiative stellar interiors. Our analysis suggests that old or less massive Ap stars have predominantly dipolar magnetic fields while more massive or younger stars exhibit more complicated field topologies. We also compared our three-dimensional chemical abundance maps of HD 24712 to the predictions of theoretical atomic diffusion calculations in magnetized stellar atmospheres, generally finding a lack of agreement between theory and observations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 39 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1349
Keyword
chemically peculiar stars, magnetic fields, spectropolarimetry, Doppler imaging, stellar atmospheres
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics; Astronomy
Identifiers
urn:nbn:se:uu:diva-278535 (URN)978-91-554-9492-6 (ISBN)
Public defence
2016-04-15, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
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Supervisors
Available from: 2016-03-21 Created: 2016-02-24 Last updated: 2016-04-04

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Rusomarov, NaumKochukhov, OlegPiskunov, Nikolai

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